1. Field of the Invention
This invention relates to a fuel arrangement for high temperature gas cooled reactors. More particularly, it provides an arrangement whereby separated directly cooled fissile and fertile fuel bearing elements are removably inserted in a plurality of prismatic moderator blocks, resulting in high coolant gas temperatures and more efficient fuel utilization.
2. Description of the Prior Art
It is well understood that the thermal efficiency of a heat cycle generally increases as the maximum temperature of the heated fluid increases. This is equally true in a nuclear reactor primary system circuit. However, unique to a nuclear reactor cycle is that in increasing overall thermal efficiency, one must also take into account those factors relating to reprocessing of the nuclear fuel resulting in maximum fuel usage. In this sense, "efficiency" refers as well to overall utilization of raw, manufactured, and recycled fissile and fertile fuels. Among the major limitations on efficient usage of nuclear fuels are those associated with reprocessing fuel containing valuable bred and unused isotopes. In the past, no arrangement has been utilized which directly facilitates non-destructive removable insertion of separate directly cooled fissile and fertile isotopes within the same fuel assembly. This is particularly true in high temperature gas cooled reactors (HTGR) which typically comprise fuel elements of coated fissile and fertile isotopes homogeneously distributed in a graphite matrix. Separation of the recycleable isotopes in such a configuration involves complex and costly techniques before full advantage can be taken of any unused or bred fuel. Although a homogeneous distribution may have short-term advantages in core power control, it has long-term disadvantages in nuclear fuel reprocessing and utilization. Another type of prior art fuel element for an HTGR is disclosed in U.S. Pat. No. 3,891,502, issued June 24, 1975 to Karl-Gerhard Hackstein et al. That patent discloses a fuel element wherein fertile and fissile fuel elements are separately disposed within a graphite moderator block. There, however, the elements are connected with the assembly graphite matrix directly and without transition, so as to contribute an integral portion of the mechanical strength of the block. Due to the necessity to contribute to the ultimate structural integrity of the block, the elements are of a continuous cross section, and cannot be directly cooled, therefore limiting the maximum safe coolant temperature that can be achieved. Further, such arrangements require the fuel elements to be mechanically drilled out of the graphite block, thereby destroying the block. Other teachings have disclosed a type of direct cooling of fuel elements, such as U.S. Pat. No. 3,738,912 issued June 12, 1973 to Lothar Rachor et al. Such teachings, however are not only based upon a homogeneous fuel distribution, but also upon mechanical means, such as specially designed spacers, to support the fuel elements within a cooling channel.
The need for power generation today is a serious concern, and commercial nuclear power plants are assuming a major worldwide role in achieving necessary electrical generation capacity. Energy needs of the future, however, may be measured not only in terms of electrical generation capacity, but also in terms of the need for synthetic liquid and gaseous fuels, fertilizers and other compounds. Hydrogen is a basic element necessary in fulfilling these needs. It is the critical element in production of ammonia for fertilizers, in iron ore reduction, in coal hydrogenation and hydrogasification, and in methanol production, among others. High temperature gas reactors, as opposed to higher pressure but lower temperature water cooled reactors, can provide a nuclear source with the capability to produce the high temperatures needed for hydrogen production and other process systems. HTGR's to date, however, have been limited to coolant temperatures for sustained long-term operation in the range of 1400.degree. to 1700.degree. F. This is basically due to maintaining safe maximum fuel temperatures and consequently rather low power densities. The low power densities further result in extremely large, massive, and costly cores and associated structures.
It is evident that a nuclear fuel arrangement which provides a very high temperature coolant without excessive fuel temperatures, and also provides non-destructable separation of fissile and fertile isotopes from the fuel assembly for reprocessing, will prove of vital importance to many existing and future worldwide energy needs.